The present invention relates to electric discharge machining methods for satinizing with a desired roughness the surface of a work piece in roll or plate form by means of electric discharges, and more particularly the invention relates to a method of controlling the feeding of an electrode to control the working gap between the electrode and a work piece at a value capable of maintaining a stable electric discharge machining condition.
In the past, methods of giving a satin finish to the surface of for example strip steel rolling rolls, particularly cold rolling rolls, have been used in which hard metal particles such as short or grit are thrown at the polished roll surfaces to form impressions in the roll surfaces, and recently attempts have been made to accomplish this type of machining by means of electric discharge machining. As is well known, the electric discharge machining is a method of machining the surface of a work piece in which an insulating liquid such as kerosene is introduced into a narrow working gap between an electrode and a work piece, and a pulse waveform voltage is periodically applied across the electrode and the work piece to cause discharges therebetween and effect the machining. By repeatedly carrying out this electric discharge machining at the roll surface while rotating the roll in the circumferential direction and simultaneously gradually moving the electrode along the rotary shaft of the roll, it is possible to continuously subject the roll surface to a spiral satin finishing operation and to form discharge impressions all over the roll surface. This is the process of uniformly giving a satin finish to the surface of a roll by means of electric discharge machining. As compared with the mechanical impressions formed by directing metal particles against the roll surface, the satin finish given by the electric discharge machining is advantageous in that the surface roughness can be adjusted as desired by changing the machining conditions, that the shape of the finished roll is extremely uniform and not affected by the manufacturing method and hardness of the roll, that the metal structure in the roll surface is hardened by the electric discharges, thus making it best suited for use as a rolling roll and so on.
With the electric discharge dull finishing methods which have heretofore been proposed, hydraulic servo control methods have been used for controlling the feeding of an electrode. In a typical method of this type, the voltage across the working gap formed by an electrode and a work piece or roll is compared with a predetermined reference voltage, and a hydraulic actuation system is controlled in response to the difference to maintain the working gap at a constant value.
However, as is well known in the art, in this type of hydraulic servo control system the electrode is repeatedly oscillated to each side of a reference position, and consequently an excessive feeding, hunting or the like is caused by inertia, thus causing the amount of machining to vary with time. As a result, while no serious problem will be caused in the case of the ordinary electric discharge machining, where the surface of a roll is machined to a slight degree while moving an electrode and the roll or work piece relative to each other, a streaky pattern will be caused in the surface of the roll by such variation with time of the amount of machining due to nonuniformity in the discharge conditions. Of course, this variation in the amount of machining is so small that the resulting streaky pattern cannot be recognized by simply looking at the finished surface with the naked eye, and the streaky pattern will be recognized only with the aid of, for example, a chalk test in which chalk powder is applied to the roll surface, or another test employing oil stone. However, if the roll is used as a strip steel rolling roll for final rolling purposes, the strip steel produced will still be affected by such streaky pattern, and the strip steel will be rejected as an off-grade product.